rgw: remove useless op ret assign

rgw: remove useless op ret assign

in op rgwsetbucketwebsite_objstore_s3 and rgwputcors_objstore_s3 get_param
trys to assign op_ret which will be overwritten by return value.
use this to be assigned value as the return value will result in
a same http response because in rgw_common rgw_http_s3_errors will translate
them into the same.

Signed-off-by: imtzw <tongzhiwei_yewu@cmss.chinamobile.com>